Method and device for determining the dispersive effect on a measurement

ABSTRACT

The invention relates to a method and to a device for detecting dispersive effects on the measurement along a line of sight according to the principle of phase or pulse modulation. The propagation times of electromagnetic radiation of at least two different carrier wavelengths are measured along a path. A first carrier wavelength in a wavelength range without molecular or atomic absorption of atmospheric gases (B) is selected and a second carrier wavelength in a wavelength range with molecular or atomic absorption, preferably in a spectral range with effective spectral lines, of atmospheric gases (B) is selected, and the dispersive effects are calculated from at least two measured propagation times of the electromagnetic radiation.

[0001] The invention relates to a method for determining the dispersive effect on a measurement along a line of sight according to the preamble of claim 1, a device for measuring the dispersion according to the preamble of claim 7, and a use of the method for the dispersive distance correction of a distance-measuring instrument according to the preamble of claim 6 and the use of a device for correcting the effects on distance measurements which result from the dispersion, as claimed in claim 17.

[0002] In electrooptical distance measurement in the distance range between 100 m and several km, the measurement is decisively influenced by the refractive index of air. The propagation velocity of an optical pulse emitted in the electrooptical distance measurement or of a signal path modulated in any desired manner is determined by the group refractive index n. This applies both to electrooptical distance-measuring instruments which are based on the phase principle and to those based on the transit time measurement principle.

[0003] Refractive index and group refractive index are not constant quantities but depend predominantly on wavelength, temperature, atmospheric pressure, gas mixture and moisture content of the atmosphere prevailing in each case.

[0004] In virtually all devices for electronic distance measurement (EDM devices), the effect of the atmospheric parameters is added as a distance correction in a further computational step after completion of the actual distance measurement. The critical atmospheric parameters are measured in each case not with the distance-measuring instrument but with other, separate instruments, such as thermometer, barometer and hygrometer.

[0005] The distance D₀ (raw measurement) directly measured and displayed on the electronic distance-measuring instrument (EDM) relates to a specific group refractive index n₀. On the basis of the additionally measured meteorological parameters comprising temperature T, atmospheric pressure p and relative humidity RH, the true group refractive index n=n(T, p, RH, . . . ) can be calculated. By means of a so-called atmospheric correction ${\Delta \quad D} = {D_{0} \cdot \left( \frac{n_{0} - n}{n} \right)}$

[0006] the true distance D can be determined:

D=D ₀ +ΔD

[0007] By means of this atmospheric “post-processing” method, as a rule accuracies of distance measurement in a region of 1 ppm are achieved; if, on the other hand, temperature T and atmospheric pressure p are not known or are not representative over the entire optical path, the measured raw distance D₀ can easily deviate by 30 ppm or more from the true value.

[0008] In the case of longer distances, which moreover generally pass over irregular topography, a reliable determination of the effective group refractive index from meteorological data is problematic at the end points of the distances. Attempts to determine these data along the target beam have not been successful to date.

[0009] One of the basic concepts is the utilization of the spectral broad-band dispersion by measuring the distance with light or electromagnetic radiation of two different wavelengths. This 2- or multicolor method of measurement has been known since about 1975. In the case of simultaneous distance measurement with at least 2 different, electromagnetic wavelengths, optically or in the microwave range, the most important atmospheric disturbance parameter or parameters can be determined by means of the known, spectral broad-band dispersive behavior of the atmosphere, in order finally substantially to correct the distance measurement for the effect of the group refractive index, which as a rule is not known exactly.

[0010] Corresponding theories are based on the spectral broad-band formulae of Edlen and Barrel & Sears. (Ref. Rainer Joeckel, Manfred Stober: Elektronische Entfernungs-und Richtungsmessung [Electronic distance and direction measurement], Verlag Konrad Wittwer.)

[0011] The results of the distance measurement of the 2 carrier wavelengths are D_(r) and D_(b), and the corresponding refractive indices are n_(r) and n_(b). The true distance is obtained by the following formula for the distance correction: $D = {D_{r} - {\left( {D_{b} - D_{r}} \right) \cdot \left( \frac{n_{r} - 1}{n_{b} - n_{r}} \right)}}$

[0012] The actual problem of this 2-color method based on the model of the spectral broad-band formula consists in the accuracy of resolution with which the difference between the distances (D_(b)-D_(r)) has to be determined. The further apart the two carrier wavelengths are, the smaller and more advantageous is the model parameter $Q = \left( \frac{n_{r} - 1}{\left( {n_{b} - n_{r}} \right)} \right)$

[0013] Since the accuracy of resolution is independent of the distance, these types of two-color instruments are potentially superior to the one-color measurement only in the case of relatively long distances substantially above 2 km.

[0014] Known 2-color instruments are, for example, Goran I from the National Physical Laboratory (Teddington, UK) with λ_(b)=458 nm and λ_(g)=514 nm, and the large Q=57. For a distance error of 1 mm, the required accuracy of resolution is 0.02 mm. Since the latter can be realized only with very great inconvenience, if at all, this method has not become established to date.

[0015] The patent U.S. Pat. No. 5,233,176 discloses a device using the 2-color method, which compensates the atmospheric effects on measurement by evaluating the deviation of two laser beams of different wavelengths from a respective reference beam path. Here, the laser light is emitted at two different carrier wavelengths in short pulses. The dispersive effect is deduced from the dispersive shift of the two beam paths from the straight line, and the measurement is corrected.

[0016] A considerable disadvantage of all devices to date which use 2 or 3 carrier wavelengths is the utilization of the slightly variable broad-band optical dispersion. The procedure is always based on the broad-band models of Barrel & Sears or on the formulae according to Edlen. Furthermore, only broad-band methods have been used to date in the microwave range too. The main disadvantage is the weak measurement effect, which permits only an inaccurate distance correction which is below the quality of the classical atmospheric correction based on the determination of the meteorological parameters T, p and RH.

[0017] It is the technical object of the present invention to provide a method and a device for measuring the dispersion along a line of sight, which are based on the utilization of spectral narrow-band, atmospheric structures, both in the absorptive (imaginary refractive index) and in the dispersive behavior (real refractive index).

[0018] This object is achieved, according to the invention, by the characterizing features of claims 1 and 7. Uses of the method and of the device are evident from claims 6 and 17. Advantageous and alternative embodiments and further developments of the devices and of the method are evident from the features of the subclaims.

[0019] On more exact consideration, it is found that the atmospheric transmission and hence also the refractive index at certain wavelengths has spectral structures which deviate in their behavior from the Edlen formula. There are therefore regions where the Edlen formula is not applicable and the effects are more pronounced than was known to date.

[0020] The meteorological, atmospheric parameter in the formula of Barrel & Sears which is dominant for the 2-color absorption method is the particle density of the air molecules, the very well fulfilled assumption of the ideal gas being made. There is therefore primarily one unknown, which can be determined by an additional measurement (the second carrier wave).

[0021] In principle, the integrated particle density of one type of molecule can be measured with a laser by means of a transmission measurement. The measured molecule must belong to the so-called uniformly mixed gases in order to be representative for the total atmospheric composition. These gases include in particular CO₂, O₂, NO₂, O₃, CH₄, NO and NH₃. In addition, the molecule must have spectral textures in the wavelength range of commercial, economical semiconductor lasers and detectors. Furthermore, the optical effect should also be reasonably measurable so that only gases having high partial pressure or strong absorption are suitable.

[0022] Oxygen O₂ can be considered to be particularly suitable. At a high concentration of 20.95%, it has its strongest absorption bands at the long-wave end of the visible spectral range between 759 nm and 778 nm. Commercially available laser diodes and also sensitive semiconductor detectors are available for this range, so that economical solutions can be realized.

[0023] The EDM received signal is measured at 785 nm and 760 nm (2 colors). In order to be able to correct the distance measurement to 1 ppm, the relative signal would have to be measured accurately to 0.1%. For a distance correction accurate to 10 ppm, an EDM signal measurement accurate to 1% is required.

[0024] The theoretically possible direct measurement of the absorption does not at present have the required resolution which however can be achieved in transit time measurements.

[0025] From scientific publications it is known that the present accuracy limit for measurements of optical signals is 1%, the main factor influencing deviations of the signal measurement being atmospheric turbulence. Thus, a distance measurement can be measured [sic] accurately at least to 10 ppm by the method according to the invention and the device according to the invention.

[0026] In comparison with the theoretically possible pure absorptive measurements, more accurate distance corrections are achievable by utilizing the dispersive effects in the region of an absorption line, not least because of the high resolution of the transit time.

[0027] Structures in the spectroscopically active oxygen band can be optimally utilized using a spectrally narrow-band single-mode diode laser. The laser is tuned to a region with an excessively high refractive index, with the result that the dispersion effect is more pronounced and the distance correction has a higher resolution.

[0028] Optimally suitable spectral ranges with up to −390 ppm deviation from the Edlen formula are blocked by strong absorption lines, the strongest group refractive index deviation with sufficiently high transmission being +48 ppm. For a measurement d_(meas,ir) in the infrared range and a measurement d_(meas,red) in the red range and with this value, the distance correction becomes:

d _(true) =d _(meas,ir) −Q·(d _(meas,ir) −d _(meas,red)) where Q=6.2.

[0029] The correction is three times better than in the case of the classical 2-color method according to Edlen or Barrel & Sears.

[0030] In this approach, however, the laser wavelength must be stabilized to the O₂ line, laser diodes and methods being known for this purpose.

[0031] Another alternative is the 2-color dispersion method using a multimode laser diode. This third method is so to speak the complementary of the classical 2-color method. The classical method employs narrow-band lasers which act on a broad-band dispersion region of the atmosphere; in this case, a broad-band light source acts on an absorption region of the air mixture which has a narrow-band structure.

[0032] In the region around 760 nm, oxygen has two strong absorption bands, one between 759.58 nm and 761.8246 nm and the other at 762.1802 nm and 778.37 nm. The first band has 115 lines, with 28 strong lines (integrated intensity> $\frac{10^{- 25}\quad {{cm}^{2} \cdot {cm}^{- 1}}}{Molecule}$

[0033] ) and the longer-wave one has 171 lines with 27 strong lines. The lines in the first band almost touch one another in the middle and are thus to a considerable extent cumulative in their optical effects on the dispersion effect.

[0034] The deviation from the formulae of Edlen or Barrel & Sears is critical for this third method. The spectrally averaged and superposed group refractive index within the absorption band is on average 13 ppm higher than the value next to the absorption band, which corresponds to the classical Edlen model. The result for the novel distance correction is:

d _(true) =d _(meas,ir) −Q·(d _(meas,ir) −d _(meas,red)) where Q=23.

[0035] In this case, the correction is just as good as in the classical 2-color method according to Edlen. However, there is the advantage that no short-wave lasers are used in the novel method. The latter are generally bulky and expensive and have a high energy consumption which are [sic] unusable for battery-operated field instruments, such as tacheometers. Short-wave laser diodes have to date had an insufficient life and reliability.

[0036] The data given must be viewed in the context of the typical atmospheric scaling errors, which may be up to or even more than 30 ppm in the case of oblique measurements, over irregular topography or in the case of large temperature differences.

[0037] A distance measurement is associated with an absolute error of ±0.3 mm even in the case of short distances. The quantity (d_(meas,ir)−d_(meas,red)) thus has at least a statistical error of about 0.5 mm. The error in the case of the 2-color distance correction Q·(d_(meas,ir)−d_(meas,red)) is therefore Q·(0.5 mm).

[0038] Alternatively, it is quite possible for the critical transmitter lying in the absorption band of oxygen to be replaced by a narrow-band LED or by an LED in combination with a narrow-band filter, for example an interference filter, by means of which the anomalous dispersion effect can also be measured, it being possible to completely dispense with the wavelength stabilization.

[0039] From the structural point of view, there are further advantages for the device according to the invention. Economical laser diodes having a small size are used as a light source in the present-day geodetic distance-measuring instruments. By means of the invention, an economical laser diode or LED can now also be used for the second light source, in particular one of the same compactness, small size and actuation behavior as the first light source of the distance-measuring instrument. Technically simple realizations of the 2-color method are therefore possible since in particular all that is necessary is to duplicate the transmitted beam path and the electronic actuation.

[0040] A further advantage is present on the receiving side. The optical carrier wavelength of the two laser diodes can be chosen to be close together. If the distance is determined, for example, as 5 nm to 10 nm, a common, simple optical bandpass filter is sufficient for background blocking.

[0041] One of the most important advantages on the receiver side is that, owing to the spectral closeness of the two carrier wavelengths, a common receiving diode, such as, for example, a customary avalanche photodiode (APD), is sufficient. In the case of the conventional 2-color methods, the optical carrier wavelengths are separated from one another by as much as possible in order to amplify the dispersion effect, but this is associated with the disadvantage that the receiving diodes do not cover this large optical spectral range.

[0042] In the realizations of the conventional 2-color method, one of the two carrier wavelengths is as a rule in the blue spectral range. In comparison with the 800 nm range, however, there are neither sensitive nor economical avalanche photodiodes in that range.

[0043] A further disadvantage of using the broad-band method with blue light is the atmospheric dispersion power. The dispersion coefficient of air increases to the power of 4 as the wavelength becomes shorter. This effect is caused by the Rayleigh dispersion mechanism. In the method of the prior art, the signal power loss in the case of the short-wave radiation is about 16 times greater than in the method according to the invention with wavelengths of about 800 nm, the blue wavelength of 400 nm is 2 times shorter than in the method according to the invention, and the factor is explained by the dependence of the Rayleigh dispersion as a function of 10 to the power of 4.

[0044] The spectral closeness of the two carrier wavelengths has even further advantages. The total optical receiving channel can be used for both colors together without particular efforts. The otherwise dominant chromatic optical image aberrations are not present. In the conventional 2-color method with carrier wavelengths lying far apart, complicated optical correction lenses have to be used in devices of the prior art.

[0045] The device according to the invention and the method are described in more detail below purely by way of example with reference to embodiments shown schematically in the drawing. Specifically,

[0046]FIG. 1 shows a use of the normal dispersion for a 2-color method of the prior art;

[0047]FIG. 2 shows a schematic diagram of the use of a device according to the invention in a theodolite telescope;

[0048]FIG. 3 shows a utilization of the narrow-band dispersion with single-mode lasers;

[0049]FIG. 4 shows a utilization of the narrow-band dispersion with multimode lasers;

[0050]FIG. 5 shows a utilization of the narrow-band dispersion with light emitting diodes (LED) and

[0051]FIG. 6 shows the fine structure curve of the group refractive index.

[0052]FIG. 1 shows a use of the normal dispersion for a 2-color method of the prior art by using two carrier wavelengths λ₁ and λ₂, which probe an atmospheric gas in the visible spectral range and in ranges of normal dispersion. The wavelength λ is plotted along the horizontal axis, and the refractive index n along the vertical axis. Since the precision of the dispersion correction requires as large a difference as possible between the measured refractive indices, it is necessary, owing to the dependence of the refractive index on the wavelength, to choose two carrier wavelengths which are far apart, i.e. the difference Δλ of the carrier wavelengths λ₁ and λ₂ is maximized.

[0053]FIG. 2 schematically shows an exemplary realization of the device according to the invention in a theodolite telescope. Radiation sources 2 which emit electromagnetic radiation of different carrier wavelengths are actuated by a transmitting unit 1. The two carrier wavelengths are combined via a beam splitter 3 and are passed through the atmospheric gas 4 to be probed onto a reflector 5. After reflection, the radiation is picked up by a receiver 6 and electronically processed in a receiving unit 7. In a down-circuit distance-measuring unit 8, the distance to be measured is calculated and is corrected for the dispersive effect. Optionally, a visual telescope 9 can be used for aligning the theodolite.

[0054]FIG. 3 shows a diagram of the method according to the invention, utilizing the narrow-band dispersion with single-mode lasers. The wavelength λ is plotted along the horizontal axis, and the group refractive index n_(g) along the vertical axis. The refractive index curve shows two resonances a of the atmospheric gas to be probed, which in this example is oxygen O₂. The method uses two laser modes 10 with carrier wavelengths λ₁, λ′₁ and λ₂ which are different but are located comparatively close together with respect to the prior art. A region of normal dispersion is probed with the carrier wavelength λ₁ or λ′₁, it being possible for the carrier wavelength λ₁, λ′₁ to be present on the shorter- or longer-wave side of the resonances a. In the case of a sufficiently large distance, it is also possible to meet a region of normal dispersion between the resonances a. The carrier wavelength λ₂ meets a region having an excessively high refractive index in the immediate vicinity of the actual resonance a. By utilizing this excessively high refractive index, a sufficiently large difference between refractive indices can also be achieved with carrier wavelengths λ₁, λ′₁ and λ₂ located close together.

[0055] An alternative form of realization is shown in FIG. 4. The wavelength λ is plotted along the horizontal axis, and the group refractive index n_(g) along the vertical axis. With two multimode lasers 11, regions of normal dispersion and regions with rotation-vibration bands are encountered. The narrow-band laser modes around the carrier wavelength λ₂ probe the R-branch R of the rotation-vibration bands of oxygen O₂ here. The other multimode laser 11 emits modes around another carrier wavelength λ₁, which in this case is, for example, in the longer-wave range of the spectrum.

[0056]FIG. 5 describes a third variant of the method according to the invention, in which the narrow-band radiation of a light emitting diode (LED) 12, which is around the carrier wavelength λ₂, overlaps the range of the R-branch R and of the P-branch P of the rotation-vibration bands of oxygen O₂ and thus completely determines it. A region of normal dispersion is covered by a second carrier wavelength λ₁, which can be realized, for example, as a single-mode laser 10. In the evaluation, the different contributions of the various wavelength-dependent refractive indices of the determined region of the R- and P-branches R, P have to be taken into account. By utilizing this region with components of excessively high refractive indices, a sufficiently large difference between the refractive indices can be achieved even with an LED and a mulitimode or single-mode laser.

[0057]FIG. 6 shows by way of example a fine-structured, narrow-band curve of the group refractive index in the absorption band of an atmospheric gas. The curve is shown only qualitatively and provides no quantitative information.

[0058] Of course, the figures shown represent one of many embodiments, and a person skilled in the art can derive alternative forms of realization, for example with the use of other atmospheric gases or other means for emission and for reception of electromagnetic radiation or for signal pick-up or signal processing. 

1. A method for determining the dispersive effect on a measurement along a line according to the principle of phase or pulse modulation, comprising means for emission (1;2) of electromagnetic radiation having at least 2 carrier wavelengths, comprising means for reception (6;7) of electromagnetic radiation of the at least 2 carrier wavelengths, comprising means for transit time determination (8) of the electromagnetic radiation, in which the following steps are carried out: measurement of transit times of electromagnetic radiation of at least two different carrier wavelengths along the line, calculation of the dispersive effect from at least 2 measured transit times of the electromagnetic radiation, wherein a first carrier wavelength is selected in a wavelength range without molecular or atomic absorption of atmospheric gases (4) and a second carrier wavelength is selected in a wavelength range with molecular or atomic absorption, preferably in a spectral range with effective spectral lines, of atmospheric gases (4).
 2. The method as claimed in claim 1, wherein at least one of the two carrier wavelengths is emitted in the wavelength range of the atmospheric transmission window for visible light or in the infrared range.
 3. The method as claimed in either of the preceding claims, wherein the second carrier wavelength is selected in a wavelength range with molecular or atomic absorption of an atmospheric gas (4), in particular of the oxygen molecule O₂.
 4. The method as claimed in any of the preceding claims, wherein, during measurement of the transit times of the electromagnetic radiation, a region with a greatly increased refractive index in the absorption band of an atmospheric gas (4) is measured using spectrally monomodal radiation and/or an absorption band of an atmospheric gas (4) is measured using spectrally multimodal radiation, optionally by shifting the wavelength over the range of the absorption band, and/or an absorption band of an atmospheric gas (4) is measured using radiation, for example of a surface- or edge-emitting LED, whose spectral width corresponds to the order of magnitude of the absorption band.
 5. A method as claimed in any of the preceding claims, wherein the means for emission (1;2) of electromagnetic radiation emit a third carrier wavelength and an absorption band of atmospheric water vapor is measured using this carrier wavelength and the dispersive effect of water vapor is eliminated.
 6. The use of the method as claimed in any of the preceding claims for the dispersive distance correction of a distance-measuring instrument.
 7. A device for determining the dispersive effect on a measurement along a line by a method as claimed in any of claims 1 to 5 according to the principle of phase or pulse modulation, comprising means for emission (1;2) of electromagnetic radiation having at least 2 carrier wavelengths, comprising means for reception (6;7) of the electromagnetic radiation having the at least 2 carrier wavelengths, comprising means for transit time determination (8) of the electromagnetic radiation along the line for radiation of the at least two different wavelengths, comprising means for determination of the dispersive effect on a measurement from at least 2 measured transit times of the electromagnetic radiation, wherein the means for emission (1;2) are formed in such a way that they emit a first carrier wavelength in a wavelength range without molecular or atomic absorption of atmospheric gases (4) and emit a second carrier wavelength in a wavelength range with molecular or atomic absorption of atmospheric gases (4), preferably in a spectral range with effective spectral lines.
 8. The device as claimed in claim 7, wherein means for calculation of a correction of the dispersive effects, for example for a distance measurement, are present.
 9. The device as claimed in claim 7 or 8, wherein the means for emission emit at least one of the two carrier wavelengths in the wavelength range of the atmospheric transmission window for visible light.
 10. The device as claimed in either of claims 7 and 9, wherein the means for emission (1;2) are formed in such a way that they emit a second carrier wavelength in a wavelength range with molecular or atomic absorption of an atmospheric gas (4), in particular of the oxygen molecule O₂.
 11. The device as claimed in any of claims 7 to 10, wherein the means for emission (1;2) are formed in such a way that they emit a second carrier wavelength in the range from 685 nm to 690 nm or 755 nm to 780 nm.
 12. The device as claimed in any of claims 7 to 11, wherein the means for emission (1;2) of electromagnetic radiation comprise at least one of the following means a laser diode for the emission of spectrally monomodal radiation, a laser diode for the emission of spectrally multimodal radiation, a laser diode for the emission of radiation whose spectral width corresponds to the order of magnitude of the absorption band, an LED for the emission of radiation whose spectral width corresponds to the order of magnitude of the absorption band, an LED-filter combination for the emission of radiation whose spectral width corresponds to the order of magnitude of the absorption band.
 13. The device as claimed in any of claims 7 to 12, wherein the means for emission (1;2) are formed in such a way that at least one carrier wavelength of the electromagnetic radiation can be shifted.
 14. The device as claimed in any of claims 7 to 13, wherein stabilizing means for at least one carrier wavelength are provided, such as Distributed Feedback (DFB), Distributed Bragg Reflector (DBR) or Fabry-Perot-Etalon locking.
 15. The device as claimed in any of claims 7 to 14, wherein the device or at least one of its components is modular.
 16. The device as claimed in any of claims 7 to 15, wherein the means for emission (1;2) of electromagnetic radiation are formed in such a way that they emit a third carrier wavelength and this carrier wavelength lies in an absorption band of atmospheric water vapor, the dispersive effect of water vapor being eliminated by a transit time measurement.
 17. The use of a device as claimed in any of claims 7 to 16 for correcting the effects, resulting from dispersion, on distance measurements.
 18. A theodolite, which comprises a device for determining the dispersive effect as claimed in any of claims 7 to
 16. 